The objectives of this research project are to assess component skills in reading and spelling, related language skills, and perceptual skills in twins selected for deficits or normal function in reading and/or ADHD. Behavior-genetic analyses will be used to assess the genetic and environmental etiology for deficits in different component reading, language, and perceptual skills. Bivariate DF analyses and confirmatory factor analyses of twin data will be used to assess the genetic and environmental covariance among the component skills. Behavior-genetic analyses will also be used to validate subtypes and/or dimensions of individual difference in reading disabled children by assessing their differential genetic and environmental etiology. Children in identified subtypes will be studied for subtype-by-treatment interactions in Research Project 5. In collaboration with research projects 1 and 4, QTL analyses will be used to assess differential genetic linkage for deficits in component reading, language, and perceptual skills. In collaboration with research project 6, the twins' patterns of brain morphology will be compared with their behavioral profiles on component reading, language, and perceptual skills.